Discrete Electrostatic Charge Transfer by the Electrophoresis of a Charged Droplet in a Dielectric Liquid

Abstract

We have experimentally investigated the electrostatic charging of a water droplet on an electrified electrode surface to explain the detailed inductive charging processes and use them for the detection of droplet position in a lab-on-a-chip system. The periodic bouncing motion of a droplet between two planar electrodes has been examined by using a high-resolution electrometer and an image analysis method. We have found that this charging process consists of three steps. The first step is inductive charge accumulation on the opposite electrode by the charge of a droplet. This induction process occurs while the droplet approaches the electrode, and it produces an induction current signal at the electrometer. The second step is the discharging of the droplet by the accumulated induced charge at the moment of contact. For this second step, there is no charge-transfer detection at the electrometer. The third step is the charging of the neutralized droplet to a certain charged state while the droplet is in contact with the electrode. The charge transfer of the third step is detected as the pulse-type signal of an electrometer. The second and third steps occur simultaneously and rapidly. We have found that the induction current by the movement of a charged droplet can be accurately used to measure the charge of the droplet and can also be used to monitor the position of a droplet under actuation. The implications of the current findings for understanding and measuring the charging process are discussed.

Crater Formation on Electrodes during Charge Transfer with Aqueous Droplets or Solid Particles

“…Analysis of the recorded video to determine the corresponding acquired charge (by balancing the electrostatic force versus the drag force) indicates that the metal ball typically receives about 70 pC to 250 pC of charge from either electrode depending on the applied field strength, while the droplet receives approximately 110 and 200 pC from the negatively and positively charged electrode respectively. The overall behavior of the droplets and metal balls, including the magnitude of charge acquired, is broadly consistent with previous studies [9,14,18,19,24]. Examination of 50-nm thick electrodes after electrically bouncing a water droplet reveals no changes visible to the naked eye.…”

Electrophoretic manipulation of multiple-emulsion droplets

“…As several other research groups before, we observed that water droplets clearly undergo discharging and re-charging with opposite polarity when making contact with a biased electrode (Hase et al, 2006;Im et al, 2012;Im et al, 2011a;Jalaal et al, 2010;Jung et al, 2008;Lee et al, 2012;Takinoue et al, 2010;Vajdi Hokmabad et al, 2012). We also observed that the initially horizontally centred oil core appears to be moving in the opposite direction of the water shell, when an electric field is applied, regardless of field direction, illustrated in Figure 1b.…”

“…For example, for a droplet radius ratio (R core /R shell ) of 0.40, the initial velocity is typically of the order of 10 -3 ms -1 , whereas the velocity after first contact is typically of the order of 10 -2 ms -1 . This general behaviour of an initial charge followed by active charging has been observed by other groups (Bailes et al, 2000;Hase et al, 2006;Im et al, 2012;Im et al, 2011a;Jalaal et al, 2010;Jung et al, 2008), although so far no other group has attempted experiments with complex-multiple droplets. As the core-shell droplet was detached from the microcapillary the inner oil droplet was horizontally centred inside the water shell (Fig.…”

Charge of water droplets in non-polar oils

“…However, the manner in which the charge can be calculated heavily depends on the hydrodynamic drag force coefficients (Ladenburg, Hadamard−Rybczynski, etc.) considered and can differ by as much as 33% [85]. This will be discussed in more detail in the Methodology chapter.…”